The present invention generally pertains to laser systems and more particularly to harmonic generation of a laser beam pulse.
It is known to use crystals to cause second harmonic generation (hereinafter “SHG”) in laser beam pulses. Such SHG crystals, however, are expensive and often require frequent replacement due to their hydroscopic nature leading to a reduction of their transparency and due to damage of the crystals over time, especially when used with high intensity lasers. Furthermore, U.S. Pat. No. 6,504,612 entitled “Electromagnetic Wave Analyzer” which issued to Trebino on Jan. 7, 2003, discusses many shortcomings of using thin SHG crystals, however, this patent teaches away from the present invention by instead employing thick SHG crystals. U.S. Pat. No. 6,504,612 is nevertheless incorporated by reference herein.
A few studies have considered third harmonic generation in hollow-core fibers, in air or in noble gases. Exemplary studies are: (a) Y. Tamaki, et al., “Phase-Matched Third-Harmonic Generation by Nonlinear Phase Shift in a Hollow Fiber,” App. Phys. B 67, 59-63 (1998); (b) S. Backus, et al., “16-fs, 1-μJ Ultraviolet Pulses Generated by Third-Harmonic Conversion in Air,” Optics Letters, Vol. 21, No. 9, 665 (May 1, 1996); and (c) C. Siders, et al., “Blue-Shifted Third-Harmonic Generation and Correlated Self-Guiding during Ultra-Fast Barrier Suppression Ionization of Subatmospheric Density Noble Gases,” J. Opt. Soc. Am. B, Vol. 13, No. 2, 330 (February 1996). The systems of these studies, however, suffer from uncorrected pulse distortions and other undesired conditions. For example, the Y. Tamaki article found that prior high-order harmonic generation in gases, such as air, led to undesirable phase mismatching conditions, due in part to self-focusing of the pulse. But, the Y. Tamaki article employed an additional hollow fiber, and the cost and complexity disadvantages associated therewith, in an attempt to overcome the prior issues. Furthermore, R. Bartels, et al., “Shaped-Pulse Optimization of Coherent Emission of High-Harmonic Soft X-Rays,” Nature, Vol. 406, 164 (2000), discloses pulse shapes characterized using the frequency-resolved optical gating (“FROG”) technique which provided only an approximation of the amplitude and phase, but not a direct measurement.